Conventional twist type drills have been used for many years, generally being formed of hardened steel. Solid carbide drills provide desired strength characteristics for machining, but have various limitations. The development of drilling tools with indexable inserts overcomes some of these limitations. Indexable drills utilize cutting edges on one or more indexable, replaceable inserts. The inserts may be seated in pockets on the cutting end of a drill body. The pockets may have a shape corresponding to at least a portion of the shape of the insert. The inserts may be indexable, meaning that when the cutting edges wear in operation, the inserts may be removed or loosened from their position on the drill body, then rotated, or indexed, a pre-determined amount to enable use of additional cutting edges on the insert. At least one cutting geometry is associated with the insert, which may be on two or more index locations, such as for example an approximately triangle shaped insert having cutting geometry at three positions, an approximately square or other quadrilateral shaped insert having cutting geometry at four positions, or other suitable shapes. Inserts may be made of a material harder and/or denser than the drill body.
Indexable inserts may be capable of cutting feeds and speeds greater than a conventional twist type drill. Inserts may be carbide materials or similar materials that have a suitable hardness or may be hardened to provide a cutting edge with a hardness greater than the material being drilled. Indexable drills may be of a “one flute effective”, “two flute effective” or multi-flute effective design, where one or more inserts are provided, each having one flute effective. Twist type drills generally have two effective flutes.
Although useful for many applications, indexable drills may not be as effective for smaller holes, for example ¾ inch (about 19 millimeters) or less as the design will typically be one flute effective. In operation, a single one flute effective indexable insert may be slower than a two flute effective twist drill or a two flute effective spade insert drills. Spade type inserts have a front “blade type” drilling point and may be of carbide construction or a fine grain tool steel.
As mentioned, indexable drills may utilize one or more replaceable inserts that are cost effective, particularly as the drill size gets larger or material gets more difficult to drill. An indexable drill may operate at a faster rotational speed, cutting greater surface area than a “spade type” drill, allowing a faster feed advancement. Another advantage of the indexable drill over a conventional spade drill, solid carbide drill, or conventional twist drill, is that the inserts are consumables. Instead of regrinding the cutting edge, the insert may be indexed to a new cutting edge and then thrown away when all the cutting edges are worn.
Indexable type drills are generally known in the art as a “roughing tool” because indexable drills may not be self-centering as in other type drilling products. Problems such as the insert flutes on an indexable not being a mirror of each other, as in spade drills or other products for example, causing non-uniform cutting forces in the operating drill. In the past, the drill may be held in the desired cutting path by a machine spindle, and the accuracy may be dependant on the rigidity of the machine and spindle, and the setup holding the part. For this reason, prior indexable drills may not be effective for drilling holes having a depth greater than 3 times the diameter. At depths greater than 3 times the diameter, the accuracy of prior art indexable drills may be reduced and the forces on the machine and machine spindle may become greater.
Indexable drills, or rough hole tooling, in the past have been made in rough hole size ranges. As the prior art drills increase in size, the difference in size from one drill to the next larger size may be about ¼ inch (about 6 millimeters) diameter or more. The end-user may have difficulty boring a hole in a single pass to a size between the prior art drill sizes. Until now, there has been little improvement in providing close drill tolerance holes in the larger drill sizes of about 1¼ inch to 5 inch (32 millimeter to 127 millimeter) and instead separate boring products are used to finish the rough hole.
Further, hole diameters between standard sizes in the past could be accommodated by special order drills. However, the lead time and expense of special order items can be high due to their low production volume, non standard size, and special engineering and sales involvement. The expense and time in obtaining these special size drills may dissuade the end user from purchasing a special drill and causing additional expense of added operations.
Also in past indexable drills, during operation, the physical cutting conditions for indexable insert drills may vary from the outside edge of the hole towards the center of the hole diameter, due to the different linear velocities of the cutting edges passing over the material. Near the center of the diameter, the linear velocity of the cutting edge may be very low. In prior indexable drills, at the low linear velocity of the center, the mechanical cutting action may be like a chisel that extrudes material. As the distance from the center increases, the linear velocity of the inserts increase, and the cutting edges may effectively shear the material. In the past, the low linear velocity at the center of the hole can cause a drill failure. To accommodate the low linear velocity at the center of the hole, drills of the prior art have attempted to utilize a center-cutting insert having a different shape, or insert material, or a special cutting edge.
Also, as the prior drills increase in size, the drill may be configured with larger sized inserts. As the insert size and shape become larger, the insert screw that retains the insert in its pocket may also change. A user having many hole sizes to create must carry a large inventory of inserts, grades of inserts, and screw sizes. The end user having holes ranging as little as 1.5 inches (38 millimeters) from the smallest to the largest, may have as many as 30 to 45 separate components to inventory.
Drills that are not balanced can cause chatter. Because indexable inserts may not be two flute effective and may not be uniform, prior indexable drills have been difficult to balance. In the past. indexable inserts have been honed or made with flat ground cutting edges to protect the edges from chatter or movement common in the prior art indexable drills. For this reason, more power was required for past drills with the honed or flat ground insert edges used in the prior art.
Indexable drills in the prior art also rely on a volume of coolant to expel the drilling chips from the hole during drilling. The chips created by the cutting action have to move back from the cutting edges and be expelled from a gullet adjacent the cutting area to keep chips from wedging in the clearance between the drill body and the cut diameter. Coolant may be pumped through the drill body, exiting on the drill end and flowing backward through pressure and momentum and pushing the chips backward up the gullet. The coolant may include a water soluble oil that may form a mist under heat and pressure of cutting. The coolant may be a health hazard to an operator when breathed and has to be contained thru filtration or other processes to reduce the airborne inhalation by machine operators.
In past indexable drills, the drill failure point may be near the center insert area. The center area may pack with chips unable to move back and over drill body structure, and out of the formed hole.